Generally, during the inspection of defects in photomasks for optical lithography in a mask defect inspection system, false defects may be induced as a result of a trade-off with calibration (for example, refer to a paragraph [0018] of Patent Document 1). Here, the term “false defect” means a non-defective element that is erroneously determined to be defective by the mask defect inspection system.
Usually, circuits are designed at a manufacturable level. Important portions in which the circuits are designed at a manufacturable level are subject to calibration in a mask defect inspection system to avoid induction of false defects.
In some cases, however, patterns exceeding, for example, a certain processing limitation may be included in a photomask in order to evaluate such some processing limitations. Such patterns would experience extraordinarily poor pattern fidelity and are not satisfactory calibrated. As a result, false defects are induced therein.
Too many false defects interrupt the operation of the mask defect inspection system. To avoid this, mask rule check (MRC) determines regions in which a large number of false defects are expected. In order to exclude the regions determined by the MRC from the inspection range, Do-Not-Inspect-Regions (DNIRs) are defined. The DNIRs are rectangular in shape.
When a plurality of DNIRs are to be defined, DNIR placement rule 1 to 5 must be obeyed (refer to Non-patent Document 1).
1. The minimum size of a DNIR is a single pixel. The pixel is a unit used in a mask defect inspection system. The length of the pixel is 125 nm, 90 nm, 72 nm or the like depending on the device or setting. The length of each pixel is also referred to as pixel size.
2. The number of DNIRs is not limited.
3. Some DNIRs may overlap with each other, and the overlapping area may be as small as it possibly be.
4. Some DNIRs may be in contact with each other by their edges.
5. If a pair of DNIRs neither overlap nor are in contact with each other, i.e., if the distance between the pair of DNIRs is not zero, the pair of DNIRs must be separated from each other by at least 128 pixels.
When a plurality of DNIRs is defined, if the DNIR placement rules are violated, an operation of the mask defect inspection system will sets off a DNIR placement rule error.
It is preferable that the size of the DNIR in the mask defect inspection system is as small as possible. However, a DNIR can be represented only by a rectangular area. When DNIRs are to be defined for a photomask illustrated in FIG. 14, for example, one DNIR may simply be defined as illustrated in FIG. 15. However, regions intended to be inspected are also included in this placement of DNIR. This problem can be solved by defining DNIRs as illustrated in FIG. 16 to reduce the size of the DNIRs. However, in this case, DNIR placement rule described above must be obeyed. When a plurality of DNIRs as illustrated in FIG. 16 are required in a photomask, conventionally, an operator has not been able to check the existence of the DNIR placement rule violation without operating the mask defect inspection system.